1. Field of the Invention
The present invention relates generally to a method for etching a chromium film formed on a substrate, and more particularly, to a method for etching a chromium film suitable for a multilayer interconnection body in which a film is further formed on the chromium film.
2. Description of the Prior Art
FIGS. 1A, 1B, and 1C are cross sectional views for explaining an example of a conventional method for etching a chromium film. FIG. 1A shows a state before etching a chromium film. In FIG. 1A, a chromium film 2 is formed on a substrate 1, and a resist pattern 3 is formed on the chromium film 2. The resist pattern 3 can be formed using a photolithographic technique ordinarily used. The chromium film 2 is then dipped in an etchant having the composition of 17 g of ammonium cerium (IV) nitrate, 5 cc of perchloric acid and 100 cc of water. This etchant is a chromium etchant used in patterning a chromium film, which is described, for example, by Naraoka and Nihei in the "Photo-Etching and Fine Structure Processing", Sogodenshi Shuppansha, issued May 10, 1977. The chromium film 2 is oxidized in the etchant, to be ions. The ions dissolve in the etchant. At this time, a portion coated with the resist pattern 3 does not come into contact with the etchant, so that the chromium film under the resist pattern 3 does not dissolve. Thus, the chromium film 2 in a portion where the resist pattern 3 is not provided dissolves, as shown in FIG. 1B. Then, as shown in FIG. 1C, the resist pattern 3 is removed by plasma ashing, so that a pattern of the chromium film 2 is obtained.
An etching end surface of the chromium film obtained by such a conventional method is approximately vertical, as shown in FIG. 1C. Therefore, when a film is further formed on this chromium film, the height of a portion corresponding to an underlayer rapidly changes, so that crystalline characteristics of the film in this portion are liable to change. Consequently, defects occur in a film formed on the chromium film, whereby the reliability is decreased.
In order to solve such a problem, Japanese Patent Laying-Open Gazette No. 111366/1983 discloses a method for etching a metal film formed on a substrate in a tapered shape. FIGS. 2A, 2B and 2C are cross sectional views for explaining the method disclosed in this gazette. FIG. 2A shows a state in which a resist is formed on a chromium film 12 duly formed on a substrate 11 and the resist is removed after the first etching. FIG. 2B shows a state in which a resist 14 is formed on the chromium film 12 shown in FIG. 2A so as to be of a pattern narrower than the pattern of the chromium film 12. The chromium film 12 is then etched while being in contact with a chromium etchant in this state. On this occasion, the chromium film 12 is not completely etched but the etching thereof is terminated in the halfway stage. Consequently, a cross section of the chromium film 12 can be made to have a gentle shape as shown in FIG. 2C. Therefore, the change in crystalline characteristics of the film formed on the chromium film can be made small, so that occurrence of defects can be suppressed and the reliability can be increased.
However, in this conventional method, the process for forming the resist pattern is required two or more times, so that the manufacturing process becomes complicated and the manufacturing cost becomes high.
Additionally, in this method, the second or the subsequent etching must be terminated halfway. Therefore, it is necessary to determine the time point when etching is terminated. However, it is difficult to determine the time point when etching is terminated, so that it is difficult to control a tapered shape of an etching end surface.